1. Field of the Invention
This invention relates to a semiconductor light-emitting device having novel element structure including an ultra-thin film active layer.
2. Description of the Related Art
In recent years, there have been successively reported silicon photonics wherein silicon materials which have been considered unsuitable for use in optical applications are used as a base material. The silicon photonics would become a basis for future super-high-speed/low-power-consumption photoelectric hybridized LSIs such as a silicon Raman laser which can be made to oscillate by optical excitation and a silicon light modulator which can be actuated at a high frequency of the order of gigahertz.
Silicon photonics is a field of research for studying how to on-chip/systematize individual photoelectronic devices constituting a basic unit of an optical interconnection composed of “emission-transmission-photodetection” by making use of LSI fabrication techniques based on CMOS interchange. At present, mainly in the United States of America, studies on the optical modulator, switches, waveguide and light-receiving device, all based on silicon, are extensively conducted. Among these schemes, the practical device that no one has succeeded to develop up to date is a current-injection-type light-emitting device which is based on silicon, especially a silicon semiconductor laser.
The semiconductor laser takes a role of a “transmitter” in a basic unit of an optical interconnection and hence is considered as being a key device. If it is possible to develop the silicon semiconductor laser, the manufacturing merit in terms of cost is considered enormous. However, in spite of many studies and researches for past many years, no one has succeeded in developing a practical silicon semiconductor laser.
Followings are the explanation about the problems on the development of silicon semiconductor laser, which will be discussed by referring to the laser oscillation conditions.
The laser oscillation conditions can be represented by a conditional inequality of: Effective gain (Γ×g)≧Loss α (where Γ is the light confinement coefficient, g is the gain, and α is the loss). It will be intuitively recognized from this conditional inequality that the oscillation of laser is enabled to generate as the effective gain is higher than the loss. Next, silicon will be compared with a compound semiconductor with respect to each of physical parameters such as Γ, g and α, thereby making it clear the problems involved in the employment of silicon.
The loss α is a total of the mirror loss of optical resonator, the absorption loss of semiconductor material, scattering loss and diffraction loss. The loss α of the optical resonator constituted by silicon can be confined, in technical viewpoint, to the same level of value as that of the optical resonator constituted by a compound semiconductor. In view of this fact, there is not any substantial difference between silicon and the compound semiconductor with respect to the loss α.
The gain “g” is an important physical parameter governing the oscillation. As the internal quantum efficiency of emission becomes higher, the value of the gain “g” becomes higher and hence is more liable to generate laser oscillation. One of the reasons which makes it possible to realize a semiconductor laser by making use of a compound semiconductor can be attributed to high internal quantum efficiency. On the other hand, silicon is an indirect semiconductor and hence silicon is non-radiative because of photon-supported transition, so that the internal quantum efficiency thereof is almost zero. This means that, before discussing the question of the laser oscillation, it should be noticed that it is difficult to realize a light-emitting device by making use of silicon to begin with.
In an attempt to enable a light-emitting device employing silicon to emit brightly, various techniques have been tried including the employment of quantum dots, ultra-thin film, rare earth element doping, dislocation engineering, semiconductor silicide, etc. However, there is still a problem that the internal quantum efficiency of emission is still too small (for example, S. Saito, et al, Jpn. J. Appl. Phys, 45, L679 [2006]; L. Pavesi, Materials Today, 8, 18 [2005]; and K. P. Homewood, Materials Today, 8, 34 [2005]).
It has been found out as a result of the studies made by the present inventors that the internal quantum efficiency of a Group IV element semiconductor such as not only silicon but also germanium and silicon germanium compound can be remarkably increased by the doping of a specific kind of impurities and by further reducing the thickness of ultra-thin film, thereby making it possible to develop the same level of strong light emission as that the compound semiconductor. Accordingly, it is possible to increase the value of g by employing a strong light emission layer having a thickness of several nanometers (impurity-doped ultra-thin silicon film) as an active layer. In view of this possibility, in the case of g, there is not any substantial difference between silicon and the compound semiconductor as in the case of α.
The light confinement coefficient Γ is a value that can be defined as a ratio of photon energy which is spatially confined in an active layer. The value of the Γ of a silicon light-emitting device is relatively low, thus clearly differentiating silicon in this respect from the compound semiconductor in contrast to the aforementioned α and g.
For example, in the case of a silicon light-emitting device of lateral current-passing structure including an active layer formed of an ultra-thin film interposed between a P- and an N-layer which are formed on an insulating layer (see for example, JP-A 2007-294628), the mode electric field distribution of the light being propagated in this light-emitting device will be concentrated not at the active layer formed of the ultra-thin silicon film but at the P- and N-layers neighboring the active layer. Because of this, the value of Γ becomes nearly zero.
Whereas, in the case of a compound semiconductor laser having an ordinary ridge structure, this Γ is enabled to have a value of several percent. This value is exceptionally large as compared with that of a silicon light-emitting device. Just like the silicon light-emitting device, in the case of the compound semiconductor laser also, an ultra-thin film such as a single quantum well or a multiple quantum well is certainly employed. However, a fundamental difference between the compound semiconductor laser and the silicon light-emitting device of lateral current-conducting structure resides in the facts that, in the case of the compound semiconductor laser, semiconductor layers (P- and N-layers) slightly differing in composition from the active layer, exhibiting a lower refractive index and a wider gap than the active layer, and having electrical conductivity exist, thereby making it possible to constitute a double hetero structure wherein the active layer is directly sandwiched by these semiconductor layers. Because of this possibility to create this double hetero structure, it is possible to confine the electric field distribution of propagation mode in the ultra-thin active layer, thus realizing an increase in value of the Γ.
In the case of the silicon light-emitting device wherein the active layer is formed of a lateral current-conducting structure consisted of an ultra-thin silicon film, it cannot take this double hetero structure for the following two reasons. One of the reasons is the lack of an appropriate semiconductor material for constituting corresponding P- and N-layers. Different from the P- and N-layers of the compound semiconductor laser, the P- and N-layers of the silicon light-emitting device are enabled to take only the role of injecting a current into the active layer formed of ultra-thin film. The other reason is the fact that even if such an appropriate semiconductor material exists, the lateral current-conducting structure is more suited for creating the active layer of “impurity-doped ultra-thin silicon film” in viewpoint of enhancing the internal quantum efficiency. In the case of the impurity-doped ultra-thin silicon film, the internal quantum efficiency can be enhanced by strengthening the electron-hole pair binding, so that it is required to employ energetically strong confinement. Because of this, excluding a very small junction portion between the active layer and the P- and N-layers for the injection of a current, most of the active layer is required to be in contact with the insulator exhibiting a wide gap.
It will be understood from the above-described explanation that mainly because of the small value of Γ, the effective gain Γg is exceptionally small as compared with that of the compound semiconductor laser. Therefore, even if it is possible to enable the silicon light-emitting device to generate the emission of light by the injection of a current, it is impossible to generate laser oscillation. In other words, even if it is possible to realize a silicon light-emitting diode, it is very difficult to realize a silicon semiconductor laser. In order to realize the silicon semiconductor laser, although it is indispensable to enhance the light confinement coefficient Γ, it is very difficult to achieve the enhancement of Γ by making use of the active layer formed of an ultra-thin silicon film of lateral current-conducting structure which is inherently difficult to realize the confinement of light.
As explained above, in the case of the conventional silicon light-emitting devices, it has been considered impossible to realize the laser oscillation by way of current injection.